Multiple transmit and receive antennas have been proposed to provide both increased robustness and capacity in next generation Wireless Local Area Network (WLAN) systems. The increased robustness can be achieved through techniques that exploit the spatial diversity and antenna gain introduced in a system with multiple antennas. The increased capacity can be achieved in multipath fading environments with bandwidth efficient Multiple Input Multiple Output (MIMO) techniques. A multiple antenna communication system increases the data rate in a given channel bandwidth by transmitting separate data streams on multiple transmit antennas. Each receiver receives a combination of these data streams on multiple receive antennas.
In order to properly receive the different data streams, receivers in a multiple antenna communication system must acquire a channel matrix through training. This is often achieved by using a specific training symbol, or preamble, to perform synchronization and channel estimation techniques. It is desirable for the multiple antenna communication system to co-exist with legacy single antenna communications systems (typically referred to as Single Input Single Output (SISO) systems) Thus, a legacy (single antenna) communications system must be able to interpret the preambles that are transmitted by multiple antenna communication systems. Most legacy Wireless Local Area Network (WLAN) systems based upon OFDM modulation comply with either the IEEE 802.11a or IEEE 802.11g standards (hereinafter “IEEE 802.11a/g”). Generally, the preamble signal seen by the legacy device should allow for an accurate synchronization and channel estimate for the part of the packet that the legacy device needs to understand.
A number of frame formats have been proposed for evolving multiple antenna communication systems, such as MIMO-OFDM systems. In one proposed frame format, the MIMO preamble is extended to include at least one long training symbol for each transmit antenna, and each transmit antenna sequentially transmits one or more long training symbols, such that only transmit antenna is active at a time. As the transmit antennas are switched on and off, the temperature of the corresponding power amplifier will increase and decrease, respectively. Generally, such heating and cooling of the power amplifier will lead to “breathing” effects that cause the transmitted signal to have a phase or magnitude offset, relative to the desired signal. It is therefore desirable to have a continuous transmission from all transmitters once the packet transmission has started to avoid temperature related signal “breathing.”
In a frame format that continuously transmits signals from all transmit branches, the same long training symbol is typically transmitted by all transmit antennas at the same time (possibly multiplied by a constant value). This scheme, however, may lead to a beamforming phenomenon if the transmitter is not surrounded by many scattering objects. In other words, when multiple antennas transmit the same signal (possible differing by a constant factor), then certain areas in the environment will be better illuminated than others. Depending on the constant factor between the various transmit antennas, these illumination areas will change.
A need therefore exists for a method and system for performing channel estimation and training in a MIMO-OFDM system that is compatible with current IEEE 802.11a/g standard (SISO) systems, allowing MIMO-OFDM based WLAN systems to efficiently co-exist with SISO systems. A further need exists for a method and system for performing channel estimation and training in a MIMO-OFDM system that does not cause beamforming effects.